1. Field of the Invention
The present invention relates to a superconducting filter device, particularly, to a superconducting filter device having an embedded bulk superconducting resonator, and a method of fabricating the superconducting filter device.
2. Description of the Related Art
In recent years and continuing, along with transition to high speed, large capacity data communications such as the next generation mobile communication system, and a wideband wireless access system, effective utilization of frequency resources becomes indispensable. A leading candidate for solving the frequency interference problem is using a high-Q superconducting filter, which has low loss and good frequency cutoff characteristics, for both signal reception and signal transmission.
A micro-strip line structure is often used in a superconducting receive filter. However, when receiving a high-power RF signal, loss in the filter increases. This is because microwaves or other high frequency signals are likely to concentrate at an edge of a conductor, hence electric currents are concentrated at edges or corners of the micro-strip lines, and the current density exceeds the critical current density of the superconductor.
As a candidate of a superconducting transmit filter, a disk type resonator pattern has been developed, which is able to prevent current concentration, and thus has a very uniform current density distribution. For example, Japanese Laid Open Patent Application No. 2006-101187 discloses such a technique.
In addition, attempts have been made to reduce the concentration of the current density by increasing the film thickness of a superconducting film. However, when the film thickness of the superconducting film is increased, the crystallinity of the superconducting film declines, so that the electrical surface resistance of the superconducting film does not improve as expected. A high temperature oxide superconductor thin film, such as a YBCO film, is often formed by CVD (Chemical Vapor Deposition), such as MOCVD (Metal Organic Chemical Vapor Deposition), and the crystallinity of the film declines along with growth.
On the other hand, a bulk superconducting material, which is nearly a single crystal, has recently become available, and it is reported that the bulk superconducting material is used in a bulk magnet to serve as a magnetic field generator. For example, reference can be made to “Development of Oxide Superconductor—Bulk Superconducting Material (QMG) and its Magnetic Application”, Morita et al, Nippon Steel Technical Report, No. 383 (2005), pp. 16-20.
The bulk superconducting material, which has good crystallinity close to a single crystal, is applicable to not only magnets but also various other devices, and it is a hot issue how to apply the bulk superconducting material to actual devices.